Fix the license file for the bundled Liberation fonts

[calixteman]

The LiberationSans-*.ttf files are the unmodified upstream release of Liberation Sans 1.07.4: their name table reports Version 1.07.4 and points at the Liberation Fonts license, and their glyph count, cmap coverage and advance widths match the upstream 1.07.4 build exactly.

That version is licensed under the GPL v2 with the Liberation font exception, but LICENSE_LIBERATION contains the SIL Open Font License 1.1, which only applies starting with Liberation 2.0.

So replace it with the License.txt and COPYING files shipped with the 1.07.4 release, and document in the README where the fonts come from and what needs to be updated when they're replaced.

It fixes #21746.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.10%. Comparing base (7364bca) to head (4315a4b).
⚠️ Report is 14 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21750   +/-   ##
=======================================
  Coverage   90.09%   90.10%           
=======================================
  Files         264      264           
  Lines       67234    67234           
=======================================
+ Hits        60574    60578    +4     
+ Misses       6660     6656    -4     

Flag	Coverage Δ
browsertest	66.34% <ø> (-0.01%)	⬇️
fonttest	8.99% <ø> (ø)
integrationtest	69.30% <ø> (-0.01%)	⬇️
unittest	58.23% <ø> (+0.01%)	⬆️
unittestcli	56.58% <ø> (-0.02%)	⬇️

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